1. Field of the Invention
The present invention relates to a prosthetic stem for implantation in a bone. More particularly, the present invention relates to a prosthetic hip stem (i.e., femoral stem) having a strengthening rib to increase the fatigue strength of the prosthetic stem.
2. Description of the Related Art
Orthopedic implants utilized to replace all, or a portion of, a patient""s joint (e.g., the hip) are commonly utilized to restore the use of, or increase the use of a joint which has deteriorated due to, e.g., aging, illness or injury. In the case of hip replacement, femoral components are utilized to replace a portion of the patient""s femur including, e.g., the femoral head and neck. A femoral stem is positioned within a canal of the femur and is secured thereto. The femoral stem includes a femoral neck adapted to receive a prosthetic femoral head to complete the femoral prosthesis. Prosthetic femoral stems are generally either cemented in the femoral canal or are interference fit therein.
Femoral stems may advantageously include a porous external surface to accommodate bone ingrowth or cement interdigitation. Various porous substances are utilized to coat the substantially nonporous outer surface (i.e., substrate) of a femoral stem including, e.g., wire mesh, or beaded or dimpled surfaces. For the purposes of this document, xe2x80x9csubstantially nonporousxe2x80x9d signifies a material having less porosity relative to the porous coating of a prosthetic stem.
Femoral stems are susceptible to fatigue failure after repeated loading over time. Stems having a porous coating are generally not as strong as a similarly sized stem absent a porous coating since, e.g., the substantially nonporous core of a coated stem (which is stronger than the porous coating) is smaller than the substantially nonporous core of a similarly sized stem absent porous coating. Furthermore, the porous coating of a femoral stem creates sharp corners between the porous coating and the substrate of the femoral stem. These sharp corners cause stress risers which can weaken the stem.
What is needed in the art is a femoral stem having a structure which increases the fatigue strength of a femoral stem having a porous coating without increasing the external dimensions thereof.
The present invention provides an improved prosthetic stem for implantation in a bone. Specifically, the present invention provides a prosthetic femoral stem having a strengthening rib protruding from the substrate thereof and being flush with any porous substance formed thereon. The strengthening rib is advantageously positioned so as to increase the cross-sectional moment of inertia of the prosthetic femoral stem and thereby increase the fatigue strength of the prosthetic femoral stem. The strengthening rib of the current invention increases the fatigue strength of the femoral stem not only by increasing the effective core area at a critical area of the stem (i.e., a high stress area where fatigue failure is likely to occur), but also by decreasing the stress risers associated with a porous coating at the aforementioned critical area. The strengthening rib is formed from a substantially nonporous material, and, in one exemplary embodiment is formed from a material substantially identical to the substrate material.
The invention, in one form thereof, comprises a prosthetic stem for implantation in a bone. The prosthetic stem of this form of the current invention includes a neck connected to a shaft, with the neck and shaft forming an obtuse angle. A porous substance protrudes outwardly from a substrate of the prosthetic stem and a substantially nonporous protrusion also protrudes outwardly from the substrate. The substantially nonporous protrusion is substantially flush with the porous substance so that the nonporous protrusion does not increase the external dimensions of the prosthetic stem.
The invention, in another form thereof, comprises a prosthetic hip stem for implantation in a femur utilizing bone cement to form a mantle about the portion of the hip stem inserted into the canal in the femur. The hip stem of this form of the current invention includes a neck connected to a shaft, with the neck extending from a medial side of the hip stem to form an obtuse angle with the shaft. A transition section is positioned intermediate the neck and the shaft and has a transverse cross-sectional area larger than the transverse cross-sectional area of the shaft. A protrusion is positioned on the external surface of the hip stem and has a height whereby the protrusion is covered by the mantle of bone cement when the prosthetic hip stem is implanted in a femur.
The present invention advantageously increases the fatigue strength of a prosthetic femoral stem without increasing the external dimensions thereof.